The present invention relates to a method and a apparatus for evaluating a cornering stability of a wheel that determine whether or not the vehicle having wheels with tires mounted thereto has a margin in which the wheel keeps no sliding and beyond which the wheel start sliding, when the vehicle is cornering.
The cornering property is a key factor for the driveability of automobile vehicles. In order to design automobile vehicles or wheels that achieve higher driveability, it is important to evaluate the cornering property accurately. It has been known that the cornering forces applied to the wheel and the slip angle of the wheel during cornering are significantly related to the cornering property.
When an automobile vehicle is cornering, an equilibrium state is maintained between the sum of the forces towards the center of the cornering line (cornering forces) generated on the contact surfaces of the front and rear tires, and the centrifugal force of the vehicle. If such an equilibrium state is broken, and the centrifugal force of the vehicle exceeds sum of the generated cornering forces, the wheel makes a large slide and reaches a slip state. In the description, the slip state means a state in which the wheel slides to such a large extent that the driveability of the vehicle by the driver is extremely deteriorated. For an automobile vehicle provided with wheels to which tires are assembled, various factors are involved in its cornering property, including vehicle structure properties (e.g., weight, balance), suspension properties, tire properties, and road surface conditions. Therefore, the traveling conditions (slip conditions) in which a lateral sliding occurs in the wheels are specific to particular vehicles or tires. Knowing the cornering stability (a margin in which the wheel keeps no sliding and beyond which the wheel start sliding (safety margin to reach slip state) ) of the wheel, as the cornering property of the automobile vehicle, under various traveling conditions is particularly important in designing automobile vehicles or wheels that achieve higher driveability.
As an example of methods of evaluating the cornering forces generated on a specific tire, a method that uses known indoor cornering test devices (e.g.,drum type test device or flatbelt type test device) has been described. In the description of the indoor cornering test devices, a specific tire is made in contact with a simulated road surface with the specific tire being loaded. Then, the specific tire is rotated while moving the virtual road surface and the specific tire relatively to each other, and the cornering forces generating on the contact surface are measured.
However, cornering forces generating on a tire which is actually mounted to a vehicle are affected by various factors, such as vehicle structure properties (e.g., weight, balance), suspension properties, tire properties, and road surface conditions as described above. Moreover, during actual traveling of the vehicle, changes in the load applied to the wheel occur more frequently due to changes in, such as, postures of the vehicle. Known indoor cornering test devices have a limitation in accuracy in reproducing various conditions the vehicle may encounter during actual traveling on the road surface (e.g., condition of load applied to the tire, tire rotating condition), thus preventing accurate measurement of the cornering forces generating on a contact surface of a specific tire.
On the other hand, as a method for evaluating cornering forces generating on a tire actually mounted to a vehicle, JP 2004-512207 A describes a system for estimating forces on a tire. The system described in JP 2004-512207 A uses a sensor to measure a torsional deformation of the side wall of the tire. Based on the measured torsional deformation, cornering forces that may generate on the tire are estimated. Also, JP 8-198131 A describes a vehicle state estimating apparatus in which a tire model corresponding to a vehicle movement state during traveling is established, and based on the tire model, a traveling state of the vehicle is estimated. According to the description of JP 8-198131 A, by using the vehicle state estimating apparatus described in JP 8-198131 A, the vehicle traveling state can be estimated accurately and constantly during the traveling.
However, for the system described in JP 2004-512207 A, it is necessary to obtain in advance a relationship between tire deformation and forces generating at the tire deformation. Obtaining the relationship between tire deformation and forces generating at the tire deformation requires a great deal of work. And moreover, if the relationship between tire deformation and forces generating at the tire deformation is used alone, it is impossible to accurately estimate the lateral forces generated on the contact surface under various conditions associated with the vehicle during actual traveling on the road (load condition applied to the tire, tire rotation state).
For the apparatus described in JP 8-198131 A, in order to estimate the actual traveling state of the wheel, it is necessary to establish a tire model as detail as possible. Therefore, prior to estimating the vehicle traveling state, there are various computing routines need to be executed, such as a characteristic curve identification routine for identifying a characteristic curve on vehicle load, and a tire model establish routine for establishing tire models. In these routines, a huge amount of data need to be acquired and subjected to computing processing, and evaluation also requires a significant amount of time and cost.
In addition, if evaluating a wheel cornering stability (a safety margin to reach slip state) under a traveling condition that has possibility of tire slipping, it is necessary to estimate the vehicle behavior in a time period as short as possible. However, the vehicle condition estimating apparatus described in JP 8-198131 A, performs analysis and processing using a tire model, after computing a slip angle of the vehicle based on the results detected by sensors mounted on the vehicle, such as a steering angle sensor, an acceleration sensor, and a yaw rate sensor. Due to such time consuming complicated processes, it is impossible to perform the computing and analysis preceding to the vehicle's behavior. It is difficult for the vehicle condition estimating apparatus described in JP 8-198131 A to evaluate the vehicle cornering stability (a safely margin to reach slip state) when the vehicle is actually cornering.